Vehicular airbag systems employ a stored deflated airbag which is adapted to be inflated on an undesirable predetermined impact of the vehicle with another object. An inflator, such as a hybrid, fluid filled, cold gas or similar type inflator, charged with an inflating fluid under pressure, such as an inert gas like argon, helium and the like or a mixture thereof, is caused to be discharged into the stored deflated airbag for inflating the same on the aforesaid undesirable predetermined impact. Accordingly, for the system to operate properly, the inflator pressure must not increase nor decrease beyond predetermined amounts, regardless of the temperature. Since the operating temperature of a vehicle varies, for example, from a vehicle parked outside in sub-freezing temperatures to one parked in the desert at elevated temperatures (greenhouse effect), the inflator pressure must be carefully monitored to maintain the integrity of the airbag systems.
One method of monitoring the pressure in an inflator is to use an ultrasonic transducer, e.g. U.S. Pat. Nos. 3,942,381 or 5,016,474, in which ultrasonic signals are supplied to an inflator container from the transducer and the time intervals between the application of such signals and the receipt of the return or echo signals from an opposing wall of the inflator are measured such as by time of flight (TOF) of the signals in order to determine various physical properties of the inflator container. One of the problems associated with such systems resides in the fact that inflator wall thickness has variable, not very tightly controlled, tolerances which adversely affect the TOF of the signals. Also, if one desires to operate the measuring system from an automotive electrical system restrictions on the available power from automotive electrical systems for driving the ultrasonic transducer would limit the distance which can be traversed by the ultrasonic waves to provide meaningful reflected signals needed to measure the gas pressure in the inflator. In addition, due to the strong influence of temperature on inflator pressure, some simple form of temperature compensation is required because any pressure measurement without such compensation would be meaningless due to the wide range of operating temperatures to which the measurement system can be subjected.